Automatic transmissions, which include automated shift, multi-ratio automatic transmissions that may be shifted under power-train load or with interruption of load, or variable-ratio transmissions, are operated by driver input via a selector lever and/or a programmed shifter, and an electronic control unit that communicates with other electronic power-train control units, or with other vehicle components such as, for example, a drive motor or a braking system, and selects and activates a suitable drive ratio.
In order to recognize failure of electronic and/or mechanical components and to react to them accordingly, motor vehicles with automatic transmissions are usually equipped with a device allowing emergency operation, that monitors pre-determined operating parameters and, upon recognition of a failure, i.e., a deviation of an observation unit or component or a software function, activates an alternative function, which makes it is possible to continue operation of the motor vehicle, with limited operating function, so that the vehicle may be driven under emergency conditions to a repair facility.
Such a device allowing emergency operation is known from EP 0 431 538 B1, for example, whereby a pre-determined drive ratio may be selected using a specialized magnetic valve.
Using a device allowing emergency operation known from EP 0 380 214 A2, the driver is able to select a forward or reverse drive ratio from those normally available when the conventional electronic control unit or the distribution cable fails, provided that the shift actuator is still functional.
Further, a device allowing emergency operation is known from DE 199 19 537 A1, in which the control unit assigned to the transmission is connected with an additional electronic actuation unit, so that, upon a failure, manual control of the transmission is possible via the actuation unit, whereby all available gear ratios may be selected and the vehicle may be driven even for long distances to a repair facility.
The constantly-increasing complexity of automatic transmissions presents ever greater challenges to the diagnosis or monitoring system. Also, safety requirements with so-called shift-by-wire systems are increasing greatly.
With known devices allowing emergency operation, this often leads to a conflict in goals, since, on the one hand, the driver should have the vehicle available as long as possible upon recognition of a fault, and on the other, critical safety situations may arise during extended operation under conditions of alternative functionality that are hard to manage because of system complexity, or that cannot be checked during long-term operation because of the large number of possibilities.
Even eventual indication on a display within the vehicle interior, intended to notify the driver that the system is defective, and that he should drive to a repair facility as soon as possible, cannot prevent the driver from continuing to drive the vehicle under conditions of alternative functionality, for example due to cost constraints.